Changing the per labelset config to be more generic

[alanprot]

What this PR does:
MaxSeriesPerLabelset was introduced on #5950 but the configuration format would not allow us in the future to configure more limits as "samples ingested per labelset"

This PR is just changing the configuration to be a bit more generic where we can extend to other limits in the future:

Config Before:

max_series_per_label_set:
  - label_set:
      labelName1: LabelValue1
    limit: 10
  - label_set:
      labelName1: LabelValue1
      labelName2: LabelValue2
    limit: 3

Config Now:

limits_per_label_set:
  - label_set:
      labelName1: LabelValue1
    limits:
      max_series: 10
  - label_set:
      labelName1: LabelValue1
      labelName2: LabelValue2
    limits:
      max_series: 3

We also changed the usage metric from cortex_ingester_active_series_per_labelset to cortex_ingester_usage_per_labelset and created a new metric with the configured limit: cortex_ingester_limits_per_labelset as follow:

cortex_ingester_limits_per_labelset{labelset="{label1=\"value1\"}",limit="max_series",user="1"} 10
cortex_ingester_usage_per_labelset{labelset="{label1=\"value1\"}",limit="max_series",user="1"} 2

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yeya24]

Thanks!